The minimum and maximum gravitational-wave background from supermassive binary black holes

TL;DR

This paper constrains the gravitational-wave background from supermassive binary black holes using electromagnetic observations, providing upper and lower bounds that inform the prospects for detection with pulsar timing arrays.

Contribution

It introduces a method to bound the gravitational-wave background from electromagnetic data, clarifying the tension between observations and theoretical models.

Findings

Maximum background amplitude comparable to pulsar timing limits.

Candidate 3C 66B challenges the lower limit assumptions.

Accurate parameters of OJ 287 could establish a significant lower bound.

Abstract

The gravitational-wave background from supermassive binary black holes (SMBBHs) has yet to be detected. This has led to speculations as to whether current pulsar timing array limits are in tension with theoretical predictions. In this paper, we use electromagnetic observations to constrain the SMBBH background from above and below. To derive the {\em maximum} amplitude of the background, we suppose that equal-mass SMBBH mergers fully account for the local black hole number density. This yields a maximum characteristic signal amplitude at a period of one year $A_{\rm{yr}}<2.4\times 10^{-15}$, which is comparable to the pulsar timing limits. To derive the {\em minimum} amplitude requires an electromagnetic observation of an SMBBH. While a number of candidates have been put forward, there are no universally-accepted electromagnetic detections in the nanohertz band. We show the candidate 3C 66B implies a lower limit, which is inconsistent with limits from pulsar timing, casting doubt on its binary nature. Alternatively, if the parameters of OJ 287 are known accurately, then $A_{\rm{yr}}> 6.1\times 10^{-17}$ at 95\% confidence level. If one of the current candidates can be established as a bona fide SMBBH, it will immediately imply an astrophysically interesting lower limit.